1. Field of the Invention
This invention relates to self-erecting towers, to a method for raising the tower, and more particularly to a method and means for lifting heavy loads atop high towers (60-100 m) on land and offshore, such as nacelles and rotors for wind turbine generators, electric transformers on top of power poles and other tall tower applications.
2. Description of the Prior Art
Recently, the cost of wind-generated electricity has been dropping due to technological innovations and economies of scale. The average turbine size is now approximately 1.5 MW with rotor diameters ranging from 70 to 85 m. This class of turbines has on average a nacelle weight of 50 tons and a rotor weight of approximately 35 tons. To improve the economics of wind turbines, manufacturers have been designing taller towers to take advantage of greater wind energy at higher levels above ground. Manufacturers are increasingly relying on 80 and 100 m tower designs, rather than the average 60-70 m towers used previously. The cost of lifting these weights to the top of tall land based or offshore towers can be prohibitive, mainly due to the reach of conventional cranes, which require much higher crane capacity than the actual weight being lifted. This is due to the bending moment resulting from the reach of the conventional cranes.
U.S. Pat. No. 6,408,575 granted on Jun. 25, 2002 describes a self-erecting tower method utilizing a hinged gin pole construction. The heavy weights of the tower and nacelle will require massive winching and gin pole structure to lift the whole tower with the nacelle and rotor.
Another approach is described in patent U.S. Pat. No. 6,278,198 issued on Aug. 21, 2001. This approach describes a moveable carriage that slides on guide rails on the two sides of the tower. The carriage is used to lift sections of the tower and the nacelles and rotor. This method requires a constant diameter tower section, which requires added support at the base to carry higher bending loads at the bottom tower section. Other methods are detailed in U.S. Pat. No. 5,181,693 issued on Jan. 26, 1993, that utilize a hydraulic cylinder to lift nesting tower sections. U.S. Pat. No. 4,272,929 issued on Jun. 16, 1981, utilizes pulley system for lifting. U.S. Pat. No. 4,266,911 issued May. 12, 1981 utilizes a cable system for lifting.
It is desirable to provide an improved method of raising a tall land based or offshore tower such as a wind turbine tower.
It is desirable to be able to lift weights onto the top of a tall tower with crane capacity comparable to the weight lifted.
It is desirable to provide a method of raising a wind turbine tower and positioning a wind turbine on the upper end of the wind turbine tower without employing a large crane.
It is also desirable to provide an improved method and means of raising a wind turbine tower that is less dangerous and can be performed in higher winds than the prior art methods.